1. Field of the Invention
This invention relates to the isolation and modulation of a receptor molecule, which binds ADDLs and mediates the memory compromising effects of ADDLs.
2. State of the Art
Alzheimer's disease (AD) is a fatal progressive dementia that has no cure at present. Although the molecular basis of the disease is not established, the present theory implicates neurotoxins derived from amyloid beta (Aβ) peptides and in particular the 42-amino acid amyloid beta peptide (Aβ1-42). Aβ is an amphipathic peptide, the abundance of which is increased by gene mutations and risk factors linked to AD. Fibrils formed from Aβ constitute the cores of amyloid senile plaques, which have been purified from the brains of AD patients. Analogous fibrils generated in vitro are lethal to cultured neurons, albeit at high, non-physiologically relevant concentrations. Nevertheless, these findings provided the central rationale for the original “amyloid cascade hypothesis”, which invoked Aβ fibril deposition and consequent neuronal death as the cause of AD memory loss (Hardy and Higgins (1992) Science 256:184-185).
Although experimental support of a causative role for Aβ1-42 in AD is strong, a number of key observations are simply not consistent with the original amyloid cascade hypothesis. One such observation is the remarkable selectivity of neuron degeneration, which is prevalent among neurons in the entorhinal cortex and CA1 hippocampal region, slight in the hippocampal CA3 region, and virtually non-existent the cerebellum. Unfortunately, the location of Aβ plaques does not correspond to these affected regions, and the overall plaque burden does not correlate with the extent of cognitive decline. (Katzman (1988) Ann. Neurol. 23(2):138-144).
Other inconsistencies emerged from studies involving Aβ overproducing transgenic AD mouse models. For example, when Aβ-targeting monoclonal antibodies were injected peripherally, two surprising results were obtained: (1) the vaccinated mice showed rapid reversal of memory loss, with recovery evident within the first 24 hours; and (2) the cognitive benefits of vaccination accrued despite no change in senile plaque Aβ levels (Dodart et al. (2002) Nat. Neurosci 5:452-457; Kotilinek et al. (2002) J. Neurosci. 22:6331-6335). Such findings are not consistent with a mechanism for memory loss dependent on neuron death caused by amyloid fibrils.
Within the past few years, the original amyloid cascade hypothesis has been revised and updated to accommodate the role of Aβ assemblies other than fibrils. In particular, soluble, non-fibrillar oligomers of Aβ1-42 known as amyloid β-derived diffusible ligands (ADDLs) are highly potent neurotoxins, which assemble from low Aβ1-42 concentrations (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453). ADDLs are the missing links in the original amyloid cascade hypothesis, and they are capable of rapid inhibition of long term potentiation (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453; Walsh et al. (2002) Nature 416: 535-539; Wang et al. (2002) Brain Res. 924:133-140), a classic experimental paradigm for memory and synaptic plasticity. In the updated Aβ cascade hypothesis, elevated Aβ1-42 monomer incorporates either into fibrils and plaques or assembles into ADDLs, wherein these two processes are separate and distinct, as illustrated below:
amyloid senile plaque⇄Monomeric AB1-42⇄ADDLs
The memory loss that occurs in AD stems from ADDL-induced synapse failure prior to neuron death, and not from fibrils, which contribute primarily to inflammatory and oxidative AD pathology (Hardy and Selkoe (2002) Science 297:353-356).
ADDLs have been shown to be prevalent in AD brain tissue, with levels more than 70-fold higher than levels in age matched control tissue (Kayed et al. (2003) Science 300:486-489; Gong et al. (2003) Proc. Natl. Acad. Sci. USA 100:10417-10422). ADDLs also are prevalent in AD transgenic mice models (Kotilinek et al. (2002) J. Neurosci. 22:6331-6335; Chang et al. (2003) J. Mol. Neurosci. 20:305-313). Further experiments have shown important neurological properties of ADDLs. ADDLs exhibit selective toxicity to hippocampal CA1 neurons compared with CA3 neurons, and the complete absence of toxicity towards cerebellar neurons (Kim et al. (2003) FASEB J. 17:118-120). Ventricular injection of Aβ1-42 oligomers into wild-type rats results in rapid behavioral compromise, with complete recovery occurring within 24 hours (Cleary et al. (2005) Nat. Neurosci. 8:79-84). These deficits have been attributed to higher order oligomers, specifically dodecamers (12-mers) (Lesne et al. (2006) Nature 440:352-357). ADDLs bind to neurons with high specificity, and they localize to post-synaptic receptors that are present only on a subset of hippocampal neurons (Lacor et al. (2004) J. Neurosci. 24:10191-10200). This binding triggers the rapid and persistent up-regulation of the immediate early gene product arc, translation of which is activity dependent at polyribosomes localized to subsets of dendritic spines (Steward et al. (1998) Neuron 21:741-751; Guzowski et al. (2000) J. Neurosci. 20:3993-4001). More recently, ADDLs have been implicated as upstream activators of tau phosphorylation and have been shown to interfere with animal behavior at femtomolar levels (Matsubara et al. (2004) Neurobiol. Aging 25:833-841).
The reversibility of memory loss in mouse models, coupled with the neurological properties of ADDLs and their presence in the AD brain, provides strong support for the ADDL hypothesis that AD is a disease of ADDL-induced synaptic failure rather than cell death (Lambert et al. (1998) Proc. Natl. Acad. Sci. USA 95:6448-6453; Klein et al. (2001) Trends Neuroscis. 24:219-220; Selkoe (2002) Science 298:789-791). The ADDL hypothesis also nicely explains the selectivity of neuronal susceptibility in AD, by invoking a specific ADDL receptor expressed on subsets of neurons in affected regions of the brain.
Prior to the work associated with the invention disclosed herein, there has not been definitive identification of a neuronal receptor that specifically binds ADDLs and mediates the memory compromising effects of ADDLs. The invention described here is highly significant from the standpoint of therapeutic intervention, because definitive identification of the ADDL receptor will enable the discovery of new composition and methods to inhibit, regulate, and/or modulate ADDL binding to neuronal cells, thereby obviating ADDL-triggered memory compromise and neurodegeneration in AD, MCI, Down's syndrome and other ADDL-related diseases. This invention satisfies this important need and provides related advantages as well.